Who they were
Tradition, following the Shiji, identifies him as Li Er, an archivist at the Zhou court and an older contemporary whom Confucius is said to have consulted about ritual. Modern scholarship debates whether a single historical Laozi existed at all.
What they did
He is credited with the Daodejing (Tao Te Ching), a text of roughly five thousand characters. Tradition says that, departing westward through the Hangu Pass, he wrote it at the request of the gatekeeper Yin Xi; the text itself likely took shape in the 4th–3rd centuries BC. Its core ideas are the Dao (the way), wu wei (effortless, non-coercive action), and the strength of the soft and low.
Legacy
The Daodejing is the foundational text of philosophical Daoism, and Laozi was later deified in religious Daoism. The book stands among the most translated in the world.